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annotate libgsmefr/enc_lag6.c @ 122:b33f2168fdec

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doc/EFR-rationale article written
author Mychaela Falconia <falcon@freecalypso.org>
date Sat, 10 Dec 2022 08:51:01 +0000
parents e109d8752d60
children f151d6c18b31
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1 /*************************************************************************
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2 *
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3 * FUNCTION: Enc_lag6
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4 *
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5 * PURPOSE: Encoding of fractional pitch lag with 1/6 resolution.
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6 *
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7 * DESCRIPTION:
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8 * First and third subframes:
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9 * --------------------------
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10 * The pitch range is divided as follows:
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11 * 17 3/6 to 94 3/6 resolution 1/6
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12 * 95 to 143 resolution 1
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13 *
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14 * The period is encoded with 9 bits.
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15 * For the range with fractions:
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16 * index = (T-17)*6 + frac - 3;
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17 * where T=[17..94] and frac=[-2,-1,0,1,2,3]
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18 * and for the integer only range
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19 * index = (T - 95) + 463; where T=[95..143]
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20 *
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21 * Second and fourth subframes:
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22 * ----------------------------
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23 * For the 2nd and 4th subframes a resolution of 1/6 is always used,
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24 * and the search range is relative to the lag in previous subframe.
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25 * If t0 is the lag in the previous subframe then
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26 * t_min=t0-5 and t_max=t0+4 and the range is given by
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27 * (t_min-1) 3/6 to (t_max) 3/6
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28 *
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29 * The period in the 2nd (and 4th) subframe is encoded with 6 bits:
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30 * index = (T-(t_min-1))*6 + frac - 3;
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31 * where T=[t_min-1..t_max] and frac=[-2,-1,0,1,2,3]
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32 *
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33 * Note that only 61 values are used. If the decoder receives 61, 62,
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34 * or 63 as the relative pitch index, it means that a transmission
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35 * error occurred and the pitch from previous subframe should be used.
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36 *
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37 *************************************************************************/
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38
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39 #include "gsm_efr.h"
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40 #include "typedef.h"
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41 #include "namespace.h"
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42 #include "basic_op.h"
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43 #include "no_count.h"
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44 #include "codec.h"
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45
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46 Word16 Enc_lag6 ( /* output: Return index of encoding */
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47 Word16 T0, /* input : Pitch delay */
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48 Word16 *T0_frac, /* in/out: Fractional pitch delay */
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49 Word16 *T0_min, /* in/out: Minimum search delay */
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50 Word16 *T0_max, /* in/out: Maximum search delay */
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51 Word16 pit_min, /* input : Minimum pitch delay */
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52 Word16 pit_max, /* input : Maximum pitch delay */
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53 Word16 pit_flag /* input : Flag for 1st or 3rd subframe */
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54 )
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55 {
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56 Word16 index, i;
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57
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58 test ();
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59 if (pit_flag == 0) /* if 1st or 3rd subframe */
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60 {
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61 /* encode pitch delay (with fraction) */
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62
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63 test ();
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64 if (sub (T0, 94) <= 0)
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65 {
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66 /* index = T0*6 - 105 + *T0_frac */
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67 i = add (add (T0, T0), T0);
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68 index = add (sub (add (i, i), 105), *T0_frac);
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69 } else
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70 { /* set fraction to 0 for delays > 94 */
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71 *T0_frac = 0; move16 ();
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72 index = add (T0, 368);
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73 }
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74
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75 /* find T0_min and T0_max for second (or fourth) subframe */
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76
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77 *T0_min = sub (T0, 5); move16 ();
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78 test ();
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79 if (sub (*T0_min, pit_min) < 0)
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80 {
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81 *T0_min = pit_min; move16 ();
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82 }
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83 *T0_max = add (*T0_min, 9);
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84 test ();
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85 if (sub (*T0_max, pit_max) > 0)
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86 {
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87 *T0_max = pit_max; move16 ();
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88 *T0_min = sub (*T0_max, 9); move16 ();
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89 }
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90 } else
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91 /* if second or fourth subframe */
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92 {
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93 /* index = 6*(T0-*T0_min) + 3 + *T0_frac */
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94 i = sub (T0, *T0_min);
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95 i = add (add (i, i), i);
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96 index = add (add (add (i, i), 3), *T0_frac);
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97 }
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98
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99 return index;
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100 }